Re: [PATCH] xen-evtchn: Bind dyn evtchn:qemu-dm interrupt to next online VCPU

[Anoob Soman]

On 02/06/17 17:24, Boris Ostrovsky wrote:
> >   
> >   static int set_affinity_irq(struct irq_data *data, const struct cpumask *dest,
> >   			    bool force)
> > diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
> > index 10f1ef5..1192f24 100644
> > --- a/drivers/xen/evtchn.c
> > +++ b/drivers/xen/evtchn.c
> > @@ -58,6 +58,8 @@
> >   #include <xen/xen-ops.h>
> >   #include <asm/xen/hypervisor.h>
> >   
> > +static DEFINE_PER_CPU(int, bind_last_selected_cpu);
> This should be moved into evtchn_bind_interdom_next_vcpu() since that's
> the only place referencing it.

Sure, I will do it.

> Why is it a percpu variable BTW? Wouldn't making it global result in
> better interrupt distribution?

The reason for percpu instead of global, was to avoid locking. We can 
have a global variable (last_cpu) without locking, but value of last_cpu 
wont be consistent, without locks. Moreover, since irq_affinity is also 
used in the calculation of cpu to bind, having a percpu or global 
wouldn't really matter, as the result (selected_cpu) is more likely to 
be random (because different irqs can have different affinity). What do 
you guys suggest.

> > +
> >   struct per_user_data {
> >   	struct mutex bind_mutex; /* serialize bind/unbind operations */
> >   	struct rb_root evtchns;
> > @@ -421,6 +423,36 @@ static void evtchn_unbind_from_user(struct per_user_data *u,
> >   	del_evtchn(u, evtchn);
> >   }
> >   
> > +static void evtchn_bind_interdom_next_vcpu(int evtchn)
> > +{
> > +	unsigned int selected_cpu, irq;
> > +	struct irq_desc *desc = NULL;
> > +	unsigned long flags;
> > +
> > +	irq = irq_from_evtchn(evtchn);
> > +	desc = irq_to_desc(irq);
> > +
> > +	if (!desc)
> > +		return;
> > +
> > +	raw_spin_lock_irqsave(&desc->lock, flags);
> > +	selected_cpu = this_cpu_read(bind_last_selected_cpu);
> > +	selected_cpu = cpumask_next_and(selected_cpu,
> > +			desc->irq_common_data.affinity, cpu_online_mask);
> > +
> > +	if (unlikely(selected_cpu >= nr_cpu_ids))
> > +		selected_cpu = cpumask_first_and(desc->irq_common_data.affinity,
> > +				cpu_online_mask);
> > +
> > +	raw_spin_unlock_irqrestore(&desc->lock, flags);
> I think if you follow Juergen's suggestion of wrapping everything into
> irq_enable/disable you can drop the lock altogether (assuming you keep
> bind_last_selected_cpu percpu).
>
> -boris

I think we would still require spin_lock(). spin_lock is for irq_desc.

> > +	this_cpu_write(bind_last_selected_cpu, selected_cpu);
> > +
> > +	local_irq_disable();
> > +	/* unmask expects irqs to be disabled */
> > +	xen_rebind_evtchn_to_cpu(evtchn, selected_cpu);
> > +	local_irq_enable();
> > +}
> > +